This invention relates to dynamoelectric machines such as generators which have a rotor with superconducting windings.
Background to the present invention is contained in Kirtley, Jr. et al. U.S. Pat. No. 3,999,091, Dec. 21, 1976, which relates to the use of a wound damper-shield winding in a superconducting rotor. As will be more apparent from the discussion hereinafter, the present invention uses a "damper" winding but not a "damper-shield" winding as in the patent, the respective windings being located differently in relation to the field winding. Cooper et al. U.S. Pat. No. 4,152,609, May 1, 1979, shows a combined damper and shield in the form of a composite metal shell.
Superconducting generators are generally characterized by having a normally conductive air gap winding on the stator and a D.C. superconducting field winding on the rotor. While it has been recognized that it is desirable to minimize the distance between the stator armature winding and the field winding for the purpose of achieving the highest machine rating possible for the physical size of the machine, practical considerations have impeded the achievement of an extremely close spacing. These practical considerations include the need to shield the field winding from A.C. fields created by the armature winding under unbalanced and fault conditions and the need to damp mechanical oscillations which occur during a period after the occurrence of a fault. Consequently, machine designs have included a number of continuous metallic shells to shield the field winding from the A.C. fields of the armature winding under unbalanced and fault conditions and to damp mechanical oscillations which occur during the post fault period. Designers have used metallic shields because a superconductor, though lossless under D.C. conditions, has a hysteresis loss under A.C. conditions and such losses in superconducting windings must be multiplied by the specific power of refrigeration on the order of 1000 in order to reflect the true electrical consumption. That is, the expense of refrigerating the D.C. field winding to ensure that it could withstand all contemplated fault and steady-state conditions would be so great that it has been generally preferred to use metallic shells despite the fact that they tend to reduce the power density from what would be obtainable if it were not necessary to design for fault conditions.
In addition to have the benefit of past experience in superconducting generator design, present day designers also have available to them an increasingly thorough understanding of A.C. losses in superconductors so that they can be calculated with greater confidence as represented by the publications of W. J. Carr, Jr., in Journal of Applied Physics, Vol. 45, No. 2, in articles beginning on pages 929 and 935, published in 1974. In addition, over the last several years the manufactures of superconductors have further advanced the art in multi-filamentary superconductors to produce very fine filament, low-loss superconductors. The trend is continuing that commercially available superconductors may have filaments approaching about 1 micrometer in diameter. With such low-loss conductors, the possibility of new superconducting generator configurations becomes more realistic.
The above-mentioned Kirtley et al. patent represents an approach to superconducting rotors without continuous metallic shields but with fault worthiness intended to be provided by a wound damper shield winding that is located on the rotor between the field winding and the armature winding. The damper shield winding is located in the cold region of the machine and has electrically conducting leads to carry electric current from the cold region to a warm region. The leads are connected to electric current in response to voltages that appear between the leads as a consequence of transient conditions in the machine and to control the current that is thus caused to flow. The Kirtley et al. patent gives several examples of the nature of such current control circuitry.
In accordance with the present invention, a damper winding is locating inside of or among the turns of the field winding so that the field winding is more closely adjacent the air gap winding of the stator. This is contrary to the Kirtley et al. patent and also to straight-forward thinking with respect to having a damper shield winding that would be normally expected to be disposed over the element to be shielded, that is the D.C. field winding, and between that element and the other element, that is the armature winding, posing the threat to the protected element. Yet the present invention achieves both effective performance and greater power density.
The fact that the damper winding does not shield the field winding still achieves effective damping of low frequency (e.g. 2-4 Hz.) oscillations. Shielding for the purpose of preventing higher frequency (e.g. 60 Hz. or more) fields from affecting the field winding is not necessary where the field winding has low loss superconductors, such as those made up of filaments of superconductive material of about 0.1 micrometer or less in diameter. If the field winding is designed to include higher loss superconductors, such as to save cost, the arrangement preferably has a warm shield at the vacuum enclosure that is merely for shielding against high frequency flux. Such a shield may be a continuous metal shell, similar to that of U.S. Pat. No. 4,152,609, but the damper winding will be performing the damping function. The shield itself need not have the capacity to damp low frequency rotor oscillations.
Therefore, in summary, the present invention is directed to a superconducting dynamoelectric machine whose rotor has in addition to a superconducting D.C. field winding, a superconducting damper winding that is located within (that is, radially inside or on the same radius as) the field winding. The damper winding is also located on an orthogonal axis in relation to the field winding axis. That is, whereas the field winding is on a direct axis, the damper winding is on a quadrature axis. The damper winding is also connected to a warm region in which current control elements are provided. Such a current control element may be a simple resistor, although more complex arrangements may also be employed such as are presented in the current control circuitry of the Kirtley et al. patent. The result is a superconducting machine that is "shieldless" in the sense that it effectively operates without the need for continuous metallic damper shields.